In response to growing alarm over the rise of obesity among its children, California recently signed into law ambitious changes in the nutrition environment of California's elementary and middle schools. SB 19, the Pupil Nutrition, Health and Achievement Act of 2001, is scheduled to begin in 2004. Among other things, this act would effectively ban the sale of sodas, candy and other energy dense snacks during the school day in all California elementary schools and limit soda sales in middle schools. It would also promote increased physical activity and increased consumption of fruits and vegetables via school district policy changes. Scientists have a window of opportunity to conduct a randomized, controlled study of SB 19 during calendar year 2003 to assess SB 19's potential for reducing child overweight. Further benefit will result from continuing surveillance of study schools during Year 2, following enactment of SB 19 in 2004. Primary study outcomes are body composition (sex-specific growth-chart adjusted BMI) and aerobic capacity (one-mile run time). These measures are already collected annually by law at most California schools. Process measures will include school level and individual student level data. School data will include analysis of competitive foods and documented changes in nutrition and physical activity policies. Individual data will come from the Healthy Kids Survey, a biennial survey completed by most schools that includes BRFS-type items on diet, physical activity and weight control practices. Blood pressure will also be assessed. Four mixed ethnicity school districts will be randomized to SB 19 or usual practices. Baseline measures of BMI and physical fitness will have already been collected, by state law. Baseline and 12-month follow-up BMI and physical fitness measures will be collected from 2520 primary and 2500 middle school students. Analyses will include multi-level modeling as well as linear mixed model analyses and will take into account intraclass correlation. Data collected statewide exclusively at the school level will be compared to the multi-level study data in order to assess the impact of the natural experiment represented by the adoption of SB19.